Phase shift mask technology allows for much smaller features to be resolved for a given illumination wavelength than conventional Chrome-On-Glass or Attenuated Phase Shift methods. Typically small features are resolved by exposing the spaces on either side through alternating 0-degree and 180-degree phase-shifted openings on a phase mask. The ends of these fine features are normally trimmed by exposure through a second block mask, since it is 0-degree and 180-degree shapes are not normally allowed to touch one another. Large features that could be easily resolved without resorting to alternating phase shift methods are normally rendered by the block mask alone.
For polysilicon gate applications, these large features normally include large decoupling capacitors and dummy fill shapes, among others. Within macros containing a large number of critical gates, the local density of 0-degree and 180-degree shapes (openings in an otherwise opaque mask) is normally about 40-65%, while the density of block shapes (opaque regions in an otherwise clear mask) is normally 70-90%. In other regions of a design, such as otherwise-empty areas containing only dummy fill shapes, the local density of 0-degree and 180-degree shapes is nearly zero, and the density of block shapes is typically 25%. In peripheral areas containing mostly large decoupling capacitors, the 0-degree and 180-degree local density is nearly zero, and the density of block shapes is about 60-75%.
On technology development testsites, there are often companion chiplets surrounding a central product or product-like chip. For these companion chiplets, the local density of 0-degree and 180-degree shapes is very often also nearly zero, and the local density of block shapes, mostly dummy fill, is approximately 25%. As a result, the local density of 0-degree and 180-degree phase shapes varies widely across the phase shift reticle, particularly on large length scales (1 mm or larger). Similarly, the local density of block shapes varies widely across the block reticle, particularly on large length scales. Variations in the local density of shapes are known to have an adverse effect on the dimensional control of features on those masks, and these non-uniformities have been demonstrated to exhibit a strong adverse effect on the manufacturing mask bias for both phase and block reticles. Similar manufacturing difficulties arising from non-uniform density of shapes can adversely affect masks used in other two-mask lithographic processes incorporating an auxiliary trim mask, used for example to create closely-space line ends in an SRAM or other memory array.
Known solutions include a two-pass methodology for mask manufacture, wherein oversized features are trimmed in a second pass through the mask writer. This approach increases the cost and turnaround-time for the masks, has the potential to increase the defect density on those masks, and is inherently a one-way process only. That is, features that were rendered too large can be trimmed, but features that were originally rendered too small cannot be readily adjusted.